RD&D-Programme 2004 - SKB

Programme
See section 17.2.17.
18.2.16 Dissolution/precipitation of impurities
Natural impurities in the backfill material do not have any bearing on its long-term performance.
On the other hand, engineering materials and forgotten materials can have an effect on the
repository, especially those that remain in large quantities. There are two compilations of the
materials that might be left behind in the repository /18-10, 18-11/.
Conclusions in RD&D 2001 and its review
The properties of low-pH cement and its effects on a final repository were discussed at an
international meeting organized by SKB. The meeting concluded that there was a need for
further development of low-pH materials for use in the final disposal of nuclear waste. Materials
whose leachate has a pH lower than 11 are judged to have negligible effects on the buffer and
the backfill.
Programme
Grouts with relatively low pHs are being developed and tested in a project together with Posiva,
see Chapter 10.
The importance of engineering and forgotten materials will be analyzed within the framework
of SR-Can.
18.2.17 Colloid release/erosion
The risk of erosion in the backfill is greatest if water flows in a gap or channel at the roof
between the backfill and the rock, where the density of the backfill will be lowest. This can
happen if there is a gap to begin with due to insufficient compaction or piping during the
compaction phase due to high water pressure and heavy water inflow. When fully wetted, the
backfill strives to expand and consolidate the boundary zone.
Conclusions in RD&D 2001 and its review
At the low density that will probably exist in the backfill at the tunnel roof, there is a risk that
the expansion capacity of the backfill according to the repository’s specification will not be
sufficient to maintain contact with the roof and upper walls in the tunnels, whereby the risk
of erosion and considerable heterogeneity in the backfill remains. This can be avoided by
backfilling with blocks with a higher bentonite content near the tunnel roof.
Newfound knowledge since RD&D 2001
The phenomenon of piping/erosion in the backfill may occur in the two Äspö projects Backfill
and Plug Test and Prototype Repository. When these tests are terminated and excavated, it will
presumably be possible to see whether the phenomenon has occurred and what its consequences
are. Furthermore, an artificial water pressure gradient is being applied during the flow tests in
the Backfill and Plug Test. The results will show whether piping has occurred.
Programme
Piping and erosion for different backfill materials will also be studied within the project
“Backfill and Closure of Tunnels and Rock Caverns”.
238 RD&D-Programme 2004